1. Technical Field
The present invention relates to the field of improving the signal to noise ratio within a radio receiver receiving correlated signals, particularly, signals angle modulated with periodic information. In the preferred embodiment, the signal is a television signal which is frequency modulated with line and field scanned picture information.
2. Background Information
In the United States, color television broadcasts are made in accordance with National Television Systems Committee (NTSC) standards, which require that picture information be separated into two components, luminance (brightness) and chrominance (color). FIG. 1 is an amplitude-versus-frequency diagram illustrating, in simplified form, one horizontal line of a typical NTSC composite color television signal. This composite signal 110 comprises a luminance signal 112 and a chrominance signal 114. Both luminance signal 112 and chrominance signal 114 occupy a nominal bandwidth of 6 MHz, within which picture carrier 116 is located 1.25 MHz above the lower end of the band. Luminance information is modulated directly onto this picture carrier. A color subcarrier 118 is located 3.579545 MHz above picture carrier 116, and chrominance information is modulated onto subcarrier 118. (Audio information is amplitude modulated on another subcarrier 120 lying near the upper edge of the band.)
With solid state technology making inexpensive video storage available, alternatives to the frequency domain multiplexing of NTSC are now economically feasible. This is particularly timely as new broadcasting services involving direct-to-the-home transmission from high power satellites in the 12 GHz band are begining. Thus, it is both technologically and economically feasible to plan a new transmission format based upon time division multiplex of the analog components of the television signal instead of using a color-subcarrier-based system. The new format is generally referred to as Multiplexed Analog Components (MAC).
In MAC, luminance and chrominance components of each line of the video signal are time-compressed so as to transmit the components of a given line sequentially. This strategy avoids the need for a color subcarrier and maintains the components free of crosstalk and intermodulation. FIG. 2 is an amplitude-versus-time diagram illustrating one horizontal line of a typical MAC composite color television signal. A single video line of 63.5 us duration is shown. This duration is the length of a standard television line in the United States. The horizontal blanking interval (HBI) 222, in which no picture information is transmitted, is typically 10.9 us in length. The chrominance signal 224 and luminance signal 228, either of which may be time-compressed, follow the HBI. Between the chrominance signal 224 and the luminance signal 228 is a 0.28 us guard band 226, to assist in preventing interference between the two signals.
The MAC color television signal of FIG. 2 is obtained by generating conventional luminance and chrominance signals (as would be done to obtain a conventional NTSC or other composite color television signal) and then sampling and storing them separately. Luminance is sampled at a luminance sampling frequency and stored in a luminance store, while chrominance is sampled at a chrominance sampling frequency and stored in a chrominance store. The luminance or chrominance samples may then be time-compressed (by writing them into the store at their individual sampling frequency and reading them from the store at a higher frequency). A multiplexer selects either the luminance or chrominance store, at the appropriate time during the active video line, for reading thus creating the MAC signal of FIG. 2. If desired, audio samples may be transmitted during the horizontal blanking interval and are multiplexed (and optionally time-compressed) in the same manner as the video samples.
At the receiver, the MAC signal is separated into its components by a demultiplexer synchronized to the multiplexer in the transmitter. Usually, a microprocessor is used to generate the selection signal which chooses either a luminance memory or a chrominance memory for writing the incoming signal. The stored luminance and chrominance are then decompressed, if necessary, by reading them from the memories more slowly than they were written. The MAC multiplexing and demultiplexing processes are well known in the art.
Luminance and chrominance in general do not at every instant occupy the entire 6 MHz bandwidth alloted to television signals and actually have varying instantaneous frequencies within the band, and this is true whether they are transmitted in the NTSC or the MAC format. For this reason, it is possible to obtain additional noise reduction, below that available in a typical receiver equipped with a limiter, a fixed-frequency bandpass filter, and a discriminator by substituting a tunable bandpass filter for the fixed-frequency filter. The tuning signal for the filter is derived from the discriminator output and fed back to tune the filter to the center frequency of the input signal, to capture the maximum amount of information with the minimum amount of noise.
Many feedback systems have been developed for this purpose. A typical system is shown in Clayton, U.S. Pat. No. 4,101,837, assigned to the same assignee as the present application. Clayton describes a circuit having an amplitude limiter and a voltage-tuned bandpass filter in feedback relationship with the limiter. The filter is tuned to the center frequency of the IF input signal. The limiter operates as a conventional amplitude limiter in the presence of a strong input signal and as a bandpass filter having a narrow bandwidth in the presence of weak or marginal input signals. (Although Clayton shows the discriminator input to be taken from the limiter output, whereas FIGS. 3 and 4 of the present application show the discriminator input taken from the output of the tunable filter, the two limiter feedback loops --limiter and tunable filter--are equivalent.)
The problem which Clayton attempts to solve relates to the unavoidable delay associated with the reactances in the main feedback loop (from discriminator to voltage-tuned filter). At those frequencies at which this delay is equivalent to an even number of half cycles (e.g., 360.degree.), the feedback is regenerative and tends to tune the filter to the instantaneous frequency. But at those frequencies at which the delay is an odd number of half cycles (e.g., 180.degree.), the feedback tends to tune the filter away from the desired frequency. Clayton solves this problem by adjusting the delay to amount to 360+ at the chrominance subcarrier frequency, the frequency "most important to signal quality," col. 2, 11. 22-23, and by using a notch filter to eliminate from the main feedback loop those frequencies for which the delay amounts to about 180.degree..
One problem I have discovered with the prior art, however, is that the picture elements occurring 360.degree. away from each other at 3.58 MHz are not necessarily highly correlated. A given picture element may bear little relation to the picture element occurring 360.degree. earlier or later at the arbitrary frequency of 3.58 MHz. Because of this, the steering signal generated by the prior picture element for predicting the instantaneous frequency of the current picture element may also bear little relation to the actual instantaneous frequency corresponding to the current picture element. On the other hand, whether a physical picture is converted to an NTSC or a MAC format for transmission, a long as it is scanned in fields of adjacent parallel lines (line and field scanning) the amount of change from a picture element on one line to the corresponding picture element on the next line, or in the next field, or in the next frame, is very slight, due to the continuity of the physical picture itself. Therefore, television signals are highly correlated on a periodic basis, which could be a line, a field, or (in interlaced scanning) a frame. In the United States, a frame consists of 525 lines and is scanned in two interlaced fields of 262.5 lines each. In addition, because the notch filter of Clayton attenuates a good deal of luminance information, the entire incoming signal is not utilized, also causing less accurate steering than could be achieved if all luminance information were utilized.